The father was the scion of a wealthy family of landowners, while the mother was a descendant of the Prince of Carloforte and heir to great wealth, but not the noble title.

[1] He learned the art of embalming and preservation of animals and contributed to establish the University Museum of Zoology and Comparative Anatomy of Palermo.

[1] He openly professed secularist, libertarian and antifascist ideas and adhered to Freemasonry, where he held the highest degree of Worshipful Master, facts that led to his being ostracised by the fascist regime.

Theodosio De Stefani Perez mainly applied himself to the study of Hymenoptera Chalcididae and their galls, with particular attention to their relation with the host plants and the agricultural processes.

[1] He was a pioneer in the study of the Hymenoptera, but this was not confined to their external descriptions and systematics, as was the common practise of the scientists of his time.